Look for libstdc++ first on the clang install directory.

When sysroot is not set, look for libstdc++ first on the clang install
directory. Before this change if clang was installed alongside a gcc with
the same version as the system one we would select the system libstdc++.

Unfortunately this is hard to test as only the non-sysroot case is changed.

Change-Id: Ie46095c4f73728b15aa2394d73ca32abfa629346